The present invention relates to a cathode ray tube and, in particular, to a cathode ray tube including a six-pole coil and one or more deflection aiding electrodes.
Conventional cathode ray tubes (CRTs), widely utilized in television and computer displays, employ an electron gun positioned in a neck of an evacuated funnel-shaped glass bulb to direct a number of electron beams, usually three, toward the center of a glass faceplate biased at a high positive potential, e.g., 30 kilovolts (kV). A deflection yoke raster scans the electron beams across the faceplate so that phosphors on the faceplate produce light, thereby to produce an image thereon. The deflection yoke includes a plurality of electrical coils positioned on the exterior of the funnel-shaped CRT near its neck. xe2x80x9cHorizontalxe2x80x9d coils of the deflection yoke produce magnetic fields that cause the electron beams to deflect or scan from side to side and xe2x80x9cverticalxe2x80x9d coils thereof produce magnetic fields that cause the electron beams to scan from top to bottom. The deflection yoke typically acts on the electron beams only in the first few centimeters of their travel immediately after exiting the electron guns, and the electrons travel in a straight line trajectory thereafter, i.e through a substantially field-free drift region. Conventionally, the horizontal scan produces hundreds of horizontal lines in the time of each vertical scan to produce the raster-scanned image.
The depth of a CRT, i.e. the distance between the faceplate and the rear of the neck, is determined by the maximum angle over which the deflection yoke can bend or deflect the electron beams and the length of the neck extending rearward to contain the electron gun. Greater deflection angles provide reduced CRT depth.
Modern magnetically-deflected CRTs typically obtain a xc2x155xc2x0 deflection angle (referred to as 110xc2x0 deflection) and for screen diagonal sizes of about 62 cm (about 25 inches) or more are so deep that they are almost always provided in a cabinet that either requires a special stand or must be placed on a floor. For example, a 110xc2x0 CRT having an about 100 cm (about 40 inch) diagonal faceplate and a 16:9 aspect ratio, is about 60-65 cm (about 24-26 inches) deep. Increasing the maximum deflection angle so as to reduce the depth of the CRT is disadvantageous and/or impractical due to, e.g., increased power dissipation, greater temperature rise, and the higher cost.
One approach to this depth dilemma has been to seek a thin or so-called xe2x80x9cflat-panelxe2x80x9d display. Flat panel displays, while thin enough to be hung on a wall, require very different technologies from conventional CRTs which are manufactured in very high volume at reasonable cost. Thus, flat panel displays are not available that offer the benefits of a CRT at a comparable cost.
In a short depth or space saving tube, it is necessary that the three beams of electrons be converged on the screen and be symmetrical. Conventional approaches do not provide a solution to this problem.
Accordingly, there is a need for a cathode ray tube adaptable for having a depth that is less than that of a conventional CRT having an equivalent screen-size in which the electron beams are substantially symmetrical at the screen.
To this end, the tube of the present invention comprises a tube envelope having a faceplate defining a center and a periphery, and having a screen electrode on the faceplate adapted to be biased at a screen potential, a source of a beam of electrons directed to impinge on the faceplate, a deflection yoke for magnetically deflecting the beam of electrons on the faceplate at a scanning rate, phosphorescent material disposed on the faceplate for producing light in response to the beam of electrons impinging thereon, and a source of a six-pole magnetic field for focusing the beam of electrons when the beam of electrons is deflected to impinge near the periphery of the faceplate.